The goal for the GW detector upgrades in the next 2-5 years is to roughly double the sensitivity to GW inspiral signals [1]}, which translates to an improvement of the same factor of the sensitivity in the 100 Hz region. In Advanced LIGO, for instance, this sets a target of \(6.6 \times 10^{-21} \)  m\(/\sqrt{\mbox{Hz}}\)  at 100 Hz in terms of coating Brownian noise, to be compared with the current estimated level of \(1.3 \times 10^{-20}\)  m\(/\sqrt{\mbox{Hz}}\)  [2]}. The Brownian noise amplitude spectral density scales with the square root of the total loss angle of the HR coating [3]}. Theerfore the upgrade goal translates into improving the high refractive index material loss angle by at least of a factor 4. Marginal or no improvement is necessary for the low refractive index material. For this reason, most of the coating research in the Virgo and LIGO collaborations focused on finding an alternative material to replace the tantala-titania layers [4]}, [5]}, while keeping the same outstanding optical properties (absorption, scattering, etc.) of the HR coatings. The main line of research explores the co-sputtering of other oxides with tantala, following the success of co-sputtered tantala-titania coatings [6]} which allowed a reduction of the coating loss angle by about 25% [7]} with respect to tantala.
